Intrusion detection system responsive to interruption of a transmitted beam

ABSTRACT

An infrared intruder detection system operable under severe weather conditions including fog, rain and snow over great distances exceeding 500 feet in the severest weather comprises a pulsed infrared transmitter and a self-contained spaced apart self-synchronized infrared receiver. Upon interruption of the transmitted beam for a predetermined period, the receiver generates an alarm in response to the remaining ever present signals due to ambient light and thermal instrument noise. When the beam is not interrupted, a local synchronous oscillator will track the incoming signal pulses. In the presence of an intruder, the pulsed beam is no longer received and the oscillator is unable to track the resulting amplified noise signal which arises from ambient light and thermal instrument noise. This results in the generation of an error signal at the output of the demodulator to which the oscillator is coupled. By the inclusion of appropriate bandpass amplifiers, filters and integrating circuits set to respond to predetermined thresholds coupled to the output of the demodulator, the receiver is able to discriminate between an intruder and such momentary interrupts as are caused by birds, falling leaves and blowing bits of paper which otherwise might cause false alarms.

United States Patent Rose et al.

Beck [72] Inventors: Edward A. Rose, San Jose; Richard er H. Krohn, Fremont; Qonald E. 57 ABS Slater, San Jose, all of Calif. An infrared intruder detection system operable under [73] Asslgnee: optmmx Santa Clara Cahfsevere weather conditions including fo'g, rain and [22] Filed; March 25, 7 snow over great distances exceeding 500 feet in the severest weather comprises a pulsed infrared trans- PP 22,183 mitter and a self-contained spaced apart selfsynchronized infrared receiver. Upon interruption of 52 us. Cl. ..340/258 B, 250/833 the P beam a Pedetermined Peri, 51 Int. Cl. ..G08b 13/18 Fecewer gamma? an alarm 'W P the 58 Field of Search...340/258 A, 258 B, 258, 258 D; mg p'esem due when f 250/83 31R 199 325/364 419 329 mal instrument noise. When the beam is not interrupted, a local synchronous oscillator will track the incoming signal pulses. in the presence of an intruder,

the pulsed beam is no longer received and the oscilla- [56] References Cited tor is unable to track the resulting amplified noise UNITED STATES PATENTS signal which arises from ambient light and thermal in- 3 479 607 11/1969 R th ff 325,419 X strument noise. This results in the generation of an i u error signal at the output of the demodulator to which 2,979,611 4/1961 Halma ..325/329 the d m is coupled. By the inclusion f 3,534,351 /1970 l-larnden, Jr. et al...340/258 B propriate band ass amplifiers, filters and integrating: Powell X circuits et to respond to predetermined thresholdsv 5,35 5 32 Bagno 340/258 B coupled to the output of the demodulator, the receiver 1,358 7 1 1 Wilcox ..325/364 X i able to discriminate between an intruder and such 2,963,588 12/1960 Wilson ..325/364 UX momentary interrupts as are caused by birds, falling 3,516,751 6/1970 Fruengel ..250/83.3 X leaves and. blowing bits of paper which otherwise 3,528,01 1 9/ 1970 Anderson ..250/ 199 might cause false alarms. 3,050,630 8/1962 Bird ..325/329 X 2,545,503 3/1951 Tucker ..340/258 A UX 8 Claim, 3 Drawing figures FOREIGN PATENTS OR 7 954,536 4/1964 Great Britain ..340/258 A IN NARROW EAND CLIPPING gi DETECTOR a BAND LlMlTED AMPLIFEER PRE- APLIFHER CIRCUIT I AM PL! F! ER 1 a 10 L H 12 E3 SYNCHRONOUS LOW PASS E DEMODULATOR QSCiLLATOR AMPUFIER es 4 l5 :7

BNTEGRATING THRESHOLD ALARM CiRCUiT DETECTOR CIRCUH'S INTRUSION DETECTION SYSTEM RESPONSIVE TO INTERRUPTION OF A TRANSMI'I'I'ED BEAM 1 Nov. 28, 1972 v Examiner-John W. Caldwell Assistant Examiner-Marshall M. Curtis PATENTEUIIIII28 I972 3.704;4s1

PULSE I TRANSMITTER 1:1: I 53,: 131131 RECE'VER IN NARRow BAND CLIPPING ,I DETECTOR BAND LIMITED AMPLIFIER PRE- AMPLIFIER CIRCUIT AMPLIFIER I0 I I2 l3 SYNCHRONOUS Low PASS FIG. 2 oscILLAToR DEMODULATOR 'AMPL FIER (I5 l6 l7 INTEGRATING THRESHOLD ALARM CIRCUIT DETECTOR CIRCUITS (N F W INVENTORS. EDWARD A. ROSE RICHARD H. KROHN BY 0 NALD ism m y! 2 .ATTORNEYS INTRUSION DETECTION SYSTEM RFSPONSIVE TO INTERRUPTION OF A TRANSMI'I'IED BEAM BACKGROUND OF THE INVENTION Numerous intrusion systems have been prepared which use as a principal feature of operation the interruption or modulation of a beam of visible or invisible radiation which may or may not be modulated in the transmitter. The period of interrupt necessary to generate an alarm may also be predetermined depending on the particular application, expected types of intrusion in the protected space and ambient light conditions.

Those systems detecting beam modulation caused by an intruder are formed to require a plurality of photocells which must be balanced and are quite sensitive to changes in ambient conditions. These features tend to result in annoying false alarms, require extensive precautions to control ambient conditions and in general are wholly unsuitable for outdoor use. A known improvement over these prior art systems uses a modulated transmitted beam to avoid many of the stringent requirements of control over ambient conditions. One such system requires the transmitter and receiver to be coupled in some manner to provide the receiver with a reference phase signal. This requirement presents a serious and costly disadvantage particularly in installations requiring burying the reference signal cable in existing structure. Another such system uses a high energy pulse to discharge a saw-tooth oscillator in the receiver. While avoiding the use of a reference signal cable coupling the receiver to the transmitter, the necessity for maintaining a high level pulse is a disadvantage under severe weather conditions of fog, rain and snow and is further limited in restricting the' distance over which the beam can be reliably transmitted. Furthermore, in such systems using low pulse repetition rates, loss of a single pulse due to a falling leaf, or statistical fluctuations in the intervening atmosphere due to weather often result in a false alarm being generated.

SUMMARY OF THE INVENTION A principal object of the present invention is an infrared intrusion detection system operable and reliable under the most severe weather conditions of fog, rain and snow over distances exceeding as much as 500 feet.

In accordance with this object there is provided a pulsed infrared transmitter and a self-contained spaced apart self-synchronized receiver. Narrow band circuits tuned to the transmitter frequency filter and amplify the signal generated upon reception of the transmitted beam together with the noise generated by ambient light and that thermally produced inherently within the electronic circuits. The amplified signal is applied to a squaring amplifier. The output of the squaring amplifier is applied simultaneously to a demodulator and a synchronous oscillator which is coupled to the demodulator. In the absence of an intruder, the oscillator tracks the output of the squaring amplifier and no error signal is generated by the phase detector. Upon full interruption of the beam, the output of the squaring amplifier is entirely the result of the noise amplified. Since the phase of the noise typically changes rapidly with time, the oscillator is unable to track the output of the squaring amplifier and an error signal is generated.

With conventional circuits having appropriately chosen time constants the period of interruption may be limited to those interruptions typically caused by human intruders as distinguished from birds, falling leaves or blowing bits of paper. In identifying a true human intrusion the receiver employs conventional dual logic which requires both that a full beam interrupt occur, and that the full interrupt persist for at least a minimum time which is associated with human size objects. Partial interrupts or full interrupts for less than the minimum time are rejected as false stimuli.

As will be evident'the system described herein is able to operate not only under very low signal to noise conditions but has been found reliable even under conditions where the amplitude of the noise exceeds the amplitude of the received signal.

These and other objects, features and advantages of the present invention will be apparent in the detailed description and accompanying drawings in which:

FIG. 1 is a block diagram of the detection system embodying the present invention;

FIG. 2 is a block diagram of the receiver of the system of FIG. 1 embodying the present invention;

FIGS. 3a, 3b and 3c are wave forms of the output of signal and no signal conditions.

DETAILED DESCRIPTION In FIG. 1 there is shown a pulsed transmitter 1 and a receiver 2 responsive to a pulsed beam 3 of infrared radiation. Infrared is preferable in that it is invisible to the naked eye and may be generated with solid-state devices which require low power, are small in size, and economical. Other types of radiation, however, may be used.

Beam 3 is typically 1 inch in diameter at its origin and typically subtends an angle of approximately 2 At a distance of 500 feet from the transmitter, beam 3 is approximately 17 feet in diameter. The receiver field of view is typically 2. Transmitter 1 is provided with a gallium-arsenide light-emitting diode which is gated at a frequency f for providing pulses of infrared radiation in the 0.90 to 0.95 micron region; this is-not visible to the human eye. While the particular frequency f that is used is not important, it is chosen to be relatively high to permit adequate band limiting as hereinafter described. 1

As shown in FIG. 2, receiver 2 comprises a detector 10 which is responsive to the radiation being transmitted by transmitter l. The output of detector 10 is passed through one or more stages of amplification 1 l, 12 and applied to a squaring amplifier l3. Amplifiers 11, 12 are hand limiting and tuned to pass a band f i 200 Hz. As will become evident hereinafter band limiting the received signal permits the receiver to track beam 3 even under conditions where the pulse amplitude of beam 3 is less than the amplitude of the ambient noise since the amplitude of the noise within the band is typically less than the amplitude of the pulses. Except for the foregoing noise rejection, all of the noise due to ambient light together with the thermal noise inherent in electronic instruments is amplified. The gain of circuit 13 and the preceding amplifiers 1 1, 12 is such that circuit 13 is always driven into saturation whether or not beam 3 has been interrupted, so that circuit 13 can be considered a clipper amplifier.

Referring to FIGS. 3a, 3b and 3c, the output of circuit 13 when beam 3 is being received is a square wave with positive and negative going pulses of equal duration, i.e. a 50 percent duty cycle, whereby circuit 13 functions as a square wave circuit in response to beam 3 being received. When beam 3 is interrupted, only the above described noise is amplified and the output or duty cycle of circuit 13 will vary and appear as a rectangular wave of positive and negative going levels of unequal duration. Depending on the positive and negative amplitude of the noise and its randomness as it swings about a predetermined potential set in amplifiers l 1, 12, the leading and trailing edges, and therefore, duty cycle of the output waveform circuit 13 will vary rapidly between the extremes illustrated in FIGS. 3b and 3c.

Turning again to FIG. 2, it will be seen that the output of circuit 13 is applied to the input of an oscillator 15 and a first one of two inputs of demodulator 16. The output of oscillator 15 is in turn coupled to the second of the two inputs of demodulator 16. Oscillator 15 may be appropriately characterized as an infinite Q circuit or as a synchronous oscillator tuned to have a bandwidth of i 300 Hz about the pulse repetition rate f, of the transmitted beam 3. Oscillator 15 is thereby adjusted to follow the slower changes in zero crossings of the duty cycle of amplifier 13 which normally occur as the pulse repetition rate f drifts due to, for example, the normal temperature differentials which develop between the transmitter l and the distant receiver 2. But because of its high Q, oscillator 15 will not track rapid changes of the duty cycle of amplifier 13 which occur, as above described, when beam 3 is interrupted. Demodulator 16 is substantially a phase detector which generates a time varying DC error signal at its output ference between the signals applied to its inputs.

To reject undesired frequency components in the output of demodulator 16, such as the f component of oscillator 15, the output of demodulator 16 is passed through a low pass amplifier 17 tuned to pass, typically 75-200 Hz which is in turn coupled to an integrating circuit 18. integrating circuit 18 includes a conventional clamp circuit, a peak-to-peak detector and an integrator which converts the time varying DC signal at its input to an integrated output signal. The output of integrating circuit 18 is applied to a threshold detector 19 which is in turn coupled to the alarm circuits.

Threshold detector 19 is adjusted to respond to a nominal threshold potential generated by the signal at its input and comprises conventional circuits of predetermined time constants. The time constants are chosen such that the threshold potential will not be reached and consequently no alarm will occur unless substantially all of the beam is interrupted for a minimum predetermined period, say T The period T, is chosen to be somewhat longer than the period during which beam 3 would be interrupted by such innocuous intruders as birds, falling leaves and flying bits of paper, but far shorter than any period of interruption arising through human intervention. v

The period T of interruption of beam 3 necessary for an alarm, however, will vary, for reasons hereinafter given, between, say T and T T still being far shorter than any period of interruption arising through human interventions. Recalling that the output of circuit 13. is

the sum of the signal and noise, it is apparent that the duty cycle, or'more specifically, the'change in duty cycle is a function of the signal-to-noise ratio. On clear, high signal-low noise, days, the duty cycle in the absence of interruption will be nearly 50 percent, the DC error signal at the output of demodulator 16 will normally be at nearly zero potential and the threshold circuits in detector 19 will require longer to charge to the threshold potential upon interruption. This length of time is found to be typically 50 milliseconds, a time far shorter than any period of interruption arising through human intervention. On foul, low signal-high noise, days, however, the situation is reversed and the average charge in detector 19 is normally close to threshold, thus the' period of interruption T for an alarm condition is shorter. This inherent increased sensitivity of the present invention during poor weather conditions is particularly advantageous since intrusions tend to occur more frequently in poor rather than clear weather.

What is claimed is:

1. An intrusion detection system comprising,

a transmitter for radiating a modulated beam of radiation, v

a receiver, spaced apart from said transmitter for receiving said modulated beam, said receiver including:

a detector adapted to detect said beam,

a circuit responsive to said detector for deriving waves with a substantially uniform duty cycle in response to said modulated beam being received and of random rapidly varying duty cycle in response to said beam being interrupted,

means connected to receive the output of said circuit for tracking the nearly uniform duty cycle waves derived from said circuit and not tracking the rapidly varying random duty cycle of said waves, said tracking means including means for deriving a further signal having a first amplitude range indicative of the duty cycle being substantially uniform and a second amplitude range, different from the first amplitude range, indicative of the duty cycle rapidly varying, and means responsive only to the second amplitude range of the further signal for deriving an indication of the beam being interrupted by an'intrusion.

2. Thesystem of claim 1 wherein said tracking means includes a relatively high-Q synchronous oscillator responsive to said waves derived by said circuit.

3. The system of claim 2 wherein said tracking means further includes demodulator means responsive to oscillations derived by the oscillator and the waves derived by said circuit for derivinganother signal having an amplitude dependent on the phase difference of said oscillations and waves. I

4. The system of claim 3 wherein said tracking means includes means responsive to the demodulator means for integrating the another signal to selectively derive the first and second amplitude ranges of the further signal.

5. The system of claim 1 wherein said circuit includes means for deriving said waves as' rectangular waves having leading and trailing edges dependent upon the amplitude of the received beam.

6. The system of claim 5 wherein said tracking means includes means for deriving a signal having an amplitude that is a function of the signal to noise ratio of the received beam such that the signal amplitude has a relatively low value in response to a high signal to noise 5 ratio of the received beam and a relatively high value in response to a low signal to noise ratio of the received beam.

7. The system of claim 1 wherein said tracking means includes means for deriving a signal having an am- 

1. An intrusion detection system comprising, a transmitter for radiating a modulated beam of radiation, a receiver, spaced apart from said transmitter for receiving said modulated beam, said receiver including: a detector adapted to detect said beam, a circuit responsive to said detector for deriving waves with a substantially uniform duty cycle in response to said modulated beam being received and of random rapidly varying duty cycle in response to said beam being interrupted, means connected to receive the output of said circuit for tracking the nearly uniform duty cycle waves derived from said circuit and not tracking the rapidly varying random duty cycle of said waves, said tracking means including means for deriving a further signal having a first amplitude range indicative of the duty cycle being substantially uniform and a second amplitude range, different from the first amplitude range, indicative of the duty cycle rapidly varying, and means responsive only to the second amplitude range of the further signal for deriving an indication of the beam being interrupted by an intrusion.
 2. The system of claim 1 wherein said tracking means includes a relatively high-Q synchronous oscillator responsive to said waves derived by said circuit.
 3. The system of claim 2 wherein said tracking means further includes demodulator means responsive to oscillations derived by the oscillator and the waves derived by said circuit for deriving another signal having an amplitude dependent on the phase difference of said oscillations and waves.
 4. The system of claim 3 wherein said tracking means includes means responsive to the demodulator means for integrating the another signal to selectively derive the first and second amplitude ranges of the further signal.
 5. The system of claim 1 wherein said circuit includes means for deriving said waves as rectangular waves having leading and trailing edges dependent upon the amplitude of the received beam.
 6. The system of claim 5 wherein said tracking means includes means for deriving a signal having an amplitude that is a function of the signal to noise ratio of the received beam such that the signal amplitude has a relatively low value in response to a high signal to noise ratio of the received beam and a relAtively high value in response to a low signal to noise ratio of the received beam.
 7. The system of claim 1 wherein said tracking means includes means for deriving a signal having an amplitude that is a function of the signal to noise ratio of the received beam such that the signal amplitude has a relatively low value in response to a high signal to noise ratio of the received beam and a relatively high value in response to a low signal to noise ratio of the received beam.
 8. The system of claim 7 wherein said tracking means includes means for integrating the signal having an amplitude that is a function of the signal to noise ratio. 